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Monogamy vs. Polygamy
Benefits of Monogamy In the case of Estrildid finches, a bird with a very short window for reproduction, it is more beneficial to stay with one parter because they don't have time to find another mate to reproduce with (Adkins-Regan & Tomaszycki, 2007). Pipe fish are another similar example: finding just one other mate can result in males to produce 10% fewer offspring due to an increase in the interspawning period (from 1-2 days to an average of 8.5) (Sogabe, Matsumoto, Yanagisawa, 2007). Monogamy also increases the liklihood of parental care, as less time is spent looking for other mates, and that time can be spent taking care of offspring and will increase the liklihood of survival of that offspring. While sometimes less aggression is seen in monogamous couples, the opposite can also be true. Consider the banded shrimp. These animals look like candy, with long antennae like spun sugar and white bodies and claws encircled with big red bands. They typically live in twos, feeding each other and standing guard while their partner molts. Each member of the pair, however, is ferociously aggressive toward shrimp of its own sex and will fight such individuals to the death (Judson, 2002) 90% of bird species are mostly monogamous, but may be opportunistically polygamous. (Drickamer, Vessey, Jakob, 2002). Evolution of Monogamy While several theories have been put forth, the 'short answer' is that monogamy will only evolve as a strategy for all members of the population when it is in the best interest of both males and females. An overview of the theories on evolution of monogamy: 1. The Good Wife Theory of Monogamy: A female is unable to rear offspring on her own, therefore the female will be obsessively faithful in fear of losing the male's help with the young. 2. Danger Theory of Monogamy: Mates are few and far apart; separately would make for risky business for the male 3. Pop 'em Out Theory of Monogamy: Females are prolific breeders; males don't leave because soon they'll be able to father more offspring. 4. Sociopath Theory of Monogamy: Organisms are often agressive toward any animals who is not their partner; often thought that such monogamy leads to agressiveness. 5. Mutually Assured Detruction Theory of Monogamy: When cheating or desertion by reproductive partner means destructive failure for both male and female. (Hornbills in Africa) (Judson, 2002) Costs of Monogamy Males invest very little in their gametes, and therefore success is dependant on how many eggs he fertilizes. One female has a limited number of eggs and therefore limiting himself to one female could lessen reproductive success. Females may end up mating with a less fit male, and therefore may have less fit offspring. Polygamous females may increase their chances of finding a very fit male. (Ricklefs, 2008). Benefits of Polygamy Increase genetic diversity: when female cheetahs mate with more than one male, they are increasing the genetic diversity not only of their offspring, but of the species as a whole, which can be a huge benefit for a species which is a victim of a genetic bottleneck (Gotelli, Wang, Bashir, Durant, 2007). A female's offspring may be fathered by the most fit male of the multiple males she copulates with, improving her indirect fitness. Costs of Polygamy *More time spent searching for mates *More energy spent on mating *Some males (or females) may get to mate a lot, while others don't get to mate at all. 'References' Adkins-Regan, E., Tomaszycki, M. (2007). Monogamy on the fast track. Biology Letters, 3(6), 617-619. Drickamer, L., Vessey, S., Jakob, E. (2002). Animal Behavior. New York, NY: McGraw Hill. Gotelli, D., Wang, J., Bashir, S., Durant, S. (2007). Genetic analysis reveals promiscuity among female cheetahs. Proceedings of the Royal Society of Biology, 274, 1993-2001. Judson, Olivia. (2002). Dr. Tatiana’s Sex Advice to all Creation. New York: Holt Paperbacks. Ricklefs, R. (2008). The Economy of Nature (6th ed.). New York: NY. W.H. Freeman and Company. Sogabe, A., Matsumoto, K., Yanagisawa, Y. (2007). Mate change reduces the reproductive rate of males in a monogamous pipe fish corythoichthys haematopterus: the benefit of long-term pair bonding. Ethology, 113(8), 764-771. See also mating systems.